Asbestos and asbestos containing materials (ACM), once considered a "miracle mineral" in the insulation industry, are now controversial due to their link with respiratory diseases. A survey by the U.S. Environmental Protection Agency (E.P.A.) estimates that asbestos is present in more than 30,000 public schools and 730,000 public or commercial buildings.
In the United States an industry has developed to remove offending asbestos. The Department of Labor's Occupational Safety and Health Administration (O.S.H.A.) has established strict regulations for the handling and the disposal of the waste products produced by the asbestos abatement industry.
Asbestos is a range of complex silicates which break down when subjected to temperatures in the range of 400.degree. to 900.degree. C. The process of breaking down of the fibrous structure also destroys the toxicity of the material. Heat treatment is therefore a logical approach to make asbestos-containing waste permanently non-toxic.
The disposal of medical wastes is a major environmental and public health problem. Medical wastes include microbiological wastes, pathological wastes, sharps, such as syringe needles, animal wastes, bedding and patient clothing, surgical wastes, dialysis unit wastes and other contaminated patient care and laboratory objects. Over 10,000 tons of hospital waste are generated daily in the United States alone.
Major current solutions to the medical waste problem include incineration and burial in landfills. Landfilling without prior treatment poses serious hazards of leakage and contamination of the environment. Incineration is the most widely used method of treating medical waste prior to burial.
Problems with conventional incinerators include air pollution due to particulate emission and the generation of large volumes of potentially toxic ash, which must be placed in a landfill. This ash contains large quantities of toxic metals, such as chromium, which can be subsequently leached from the buried ash by the action of groundwater and contaminate water sources. Ash may also contain incompletely oxidized carbonaceous matter which is potentially carcinogenic, and may even include surviving pathogens as a result of incomplete incineration. Incinerator emissions may be contaminated with surviving pathogens, in particular spore-forming bacteria, as well as organics, carbon monoxide, particulate matter, and acid gases.
Other methods for the treatment of medical wastes, such as sterilization by means of autoclaving or microwaving, do not offer means of dealing with sharps and immobilizing toxic metals. In addition, these methods, unlike incineration, are inapplicable to a large fraction of medical wastes, since they cannot solve the problem of disposal of pathological wastes such as human and animal body parts and are unsuitable for use with large contaminated objects. Accordingly, no completely satisfactory methods for the management of medical wastes are available at the present time.
There are also large volumes of toxic materials including contaminated soils, sludges, ash, mine tailings, etc. which cannot be readily warehoused in landfills, because there is no guarantee that they will be contained within the landfill and not leak out. These toxic wastes must be immobilized so as not to be dispersed into the environment. In addition, heavy metals, pathological vectors, pesticides, pcbs and radioactive materials can be transported from a landfill into drinking water by movement of ground water. Humans and animals may disturb a soil cap over the waste contained within a landfill, thus permitting the dispersion of the waste by wind or rain.
Vitrification has been chosen by the EPA as the best "demonstrated available technology" for high level nuclear waste, which is far more hazardous than toxic wastes. However, due to the volume of toxic wastes as compared to high level nuclear waste, it is too expensive to do extensive pretreatments to toxic waste or low level nuclear waste. Thus, there is a need to develop a robust vitrification process which can accommodate asbestos materials, ash of infectious waste, toxic materials and low level radioactive waste, which is described in the present invention.
In order to design a heat treatment process for asbestos containing waste, infectious waste ash, toxic materials and low level radioactive waste several strict standards for vitrification must be established. Such standards would necessitate that the asbestos containing waste, ash of infectious waste, toxic materials and low level radioactive waste be rendered non-toxic, or environmentally stable. The process itself also must be intrinsically safe in operation, capable of handling most forms of ACM (asbestos and associated materials), infectious waste, toxic materials, low level, TRU (transuranic waste), and high level radioactive waste, and be simple to operate.
The ACM may include pipe insulation, boiler gaskets, ceiling tile, floor tile, plaster, roofing materials, brake linings, blown insulation, wire, other reinforcing materials and any items contaminated with asbestos during asbestos abatement projects.
U.S. Pat. No. 4,678,493 and U.S. Pat. No. 4,820,328, both to Roberts et al. disclose methods of vitrification of asbestos waste. In these patents, the raw material, in this case asbestos, is placed in silos and mixed with other batch materials, such as cullet, alkalis and other fluxes. If necessary, the materials are first ground together and transferred to a furnace by a screw drive, or other means. The mixture of asbestos and other materials is fed to an electrical glass melting furnace and discharged above a body of molten glass. Premixing of the batch ingredients before melting, can in many cases cause considerable problems. Especially if the waste has cementation properties, such as in asbestos containing waste, large clumps of waste can bind together which may disrupt the feed system by jamming it.
U.S. Pat. No. 4,820,328 to Roberts et al. specifically teaches the use of an electrical furnace with temperatures preferably in the range of 1350.degree.-1380.degree. C., corresponding to a temperature of about 1250.degree. C. at the top of the molten glass. This usually requires Molybdenum electrodes, which limit the amount of oxidation potential of the melt. If the asbestos includes associated iron parts such as chicken wire or other reinforcing materials, the iron parts will not be readily consumed in a Molybdenum electrode glass furnace.
It is known that certain alkali salts, especially sulfates and chlorides such as Na.sub.2 SO.sub.4, NaCl, K.sub.2 SO.sub.4, KCl, Li.sub.2 SO.sub.4, and LiCl, when present in a mixture of components placed in a silicate glass melter, do not form a part of the glass melt upon heating but constitute a separate liquid layer, known as the gall layer, which floats on top of the heavier silicate melt. The same phenomenon occurs when alkaline earth salts, such as CaSO.sub.4, CaCl.sub.2, MgSO.sub.4, and MgCl.sub.2 are present, since these can react with alkalies and give rise to a similar gall layer. These phenomenon are described, for instance, in U.S. Pat. No. 3,499,743 to Fanica et al., or can be generated from sulfates or chlorides present in the batch of materials which require vitrification, e.g. incinerator ash, asbestos containing waste, certain toxic materials (many sludges are high in sulfate) and possibly low level radioactive waste. Conventional thinking has addressed the insoluble sulfate problem by reducing it to SO.sub.2 and letting the SO.sub.2 escape with the off-gas. However, new regulations propounded by the EPA prevent implementation of this solution because of the acid rain problem. Further, an over oxidized batch (melt) can erupt in a foaming incident when one nucleates the first oxygen bubbles. In Fanica et al., at least one rod electrode is present in this gall layer. This structure requires current across the electrodes which would rapidly degrade the electrodes. The very high current densities needed are uneconomical for the power supply.
Sorg et al. (U.S. Pat. No. 4,944,785) describes a process of using such gall layers to decompose or absorb residual hazardous components from the gas emitted by the melter. The process described by Sorg et al. makes use of high temperatures in the glass melt ranging around 1400.degree. C., which cause a substantial amount of the sulfates and chlorides to decompose and be emitted into the exhaust gas, mainly in the form of HCl and SO.sub.2. This exhaust gas is cooled to cause condensation and the condensation products are reintroduced into the batch to be melted or completely removed from the system. This limits the growth of the gall layer during continuous operation of the glass melter.
The gall layer has a much higher specific conductivity than the molten silicate. Thus, if the gall layer touches or is close to the electrodes it will effectively short them. For a given voltage difference between electrodes, the current density near or at the gall layer will be very high. In order to prevent excessive corrosion in the electrodes, one must limit the current density at the electrode. To efficiently heat the glass the current should be substantially distributed within the glass layer and not be concentrated in the gall layer.
When treating nuclear waste, close control of the redox state of the batch chemicals is maintained by the addition of nitrates/hydrocarbons. For example it has been thought that by adding sugar to a melt the number and amount of foaming incidents would be reduced. This belief is based upon the theory that foaming incidents were caused by the release of oxygen when the transition elements dropped from higher oxidation states to a lower oxidation state. However, since this procedure requires the use of reducers and the nucleation of bubbles, the melter can be overoxidized. When the process starts it could be explosive, resulting in melt choking, inlet blockage and excessive off-gas.